Causes:
La Nina is an event that, like the El Nino, alters the climate, surface temperature, and sea surface temperature. Over the Northern East Pacific, the jet stream increases significantly causing an increase in storms, precipitation, and cold temperature. On the other hand, in the southern states, there is a decrease in storms and precipitation. During La Nina, there is a failure in the formation of the Indian monsoons interfering with agricultural production. Temperatures around the equator start to decline as the easterly trade winds and the cold up welling get stronger.
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The trade wind (shown in blue) that flows toward the west cause an upwelling of cool water causes the cooling of temperature in the equatorial regions.

Credit: NOAA, NASA, Environment Canada

Effects:

When it comes to the affecting the global climate, La Niña has the opposite effects than El Nino. In the regions in the western equator, La Niña causes a cooling of temperature which often is accompanied by heavy rainfall.
La Niña most of the time bring virtually opposite effects of El Niño to the United States — wetter than normal conditions across the Pacific Northwest and dryer and warmer than normal conditions across much of the southern level. In mainland U.S. during a La Niña year, winter temperatures are warmer than normal in the Southeast and cooler than normal in the Northwest.
Like stated previously La Niña in general causes a majority of the impacts to occur in climates that have significant oceanic influences and border the tropical Pacific. The regions of the world that show the highest correlation to warm or cold events are Indonesia, Australia, and the tropical Pacific islands. La Niña can have significant impact on agricultural production such as poor crops due to failure of the Indian monsoon.
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In this picture it shows the areas that are normally dry experience precipitation during La Nina. It also shows a relatively dry climate in areas that are usually wet.


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References:

1. El niño & la niña . (n.d.). Retrieved from http://www.weatherexplained.com/Vol-1/El-Ni-o-La-Ni-a.html
2. Pielke, R. A., & Last Pielke, C. W. (n.d.). La niña, el niño, and atlantic hurricane damages in the united states . Retrieved from http://www.aoml.noaa.gov/hrd/Landsea/lanina/index.html